The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as an oligomerisation reaction, and the products are low molecular weight oligomers which are formed by the condensation of up to 12, typically 2, 3 or 4, but up to 5, 6, 7, or even 8 olefin molecules with each other. As used herein, the term ‘oligomerisation’ is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butenes, pentenes and hexenes) can be converted by oligomerisation over a zeolite catalyst, to a product which is comprised of oligomers and which is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products. Such chemical intermediates and end-products include alcohols, acids, detergents and esters such as plasticiser esters and synthetic lubricants. Industrial oligomerisation reactions are generally performed in a plurality of tubular or chamber reactors. Sulphated zirconia, liquid phosphoric acid and sulphuric acid are also known catalysts for oligomerisation.
Industrial hydrocarbon conversion processes employing zeolite catalysts typically run for several weeks before a catalyst change is required or a decommissioning of the reactor is needed. In industrial processes the feeds for the reactions are generally obtained from refining activities such as a stream derived from catalytic or steam cracking, which may have been subjected to fractionation. The nature of such refining activities is such that there will be variations in the composition of the feed. In addition it may be desired to change the nature of the feed during a reactor run. The catalyst activity and the reaction conditions vary according to the composition of the feed. Furthermore, the reactions are exothermic and the size of the exotherm also depends upon the nature and amount of olefin present in the feed. Isobutylene and propylene are particularly reactive generating a large amount of heat per unit of mass reacting.
The present invention is concerned with such processes that employ a zeolite oligomerisation catalyst in a tubular reactor and is particularly concerned with the provision of reaction conditions which enhance the conversion of the reaction.
Our copending applications PCT/EP/2005/005784, published as WO2005/118513, and PCT/EP/2005/005785, published as WO2005/118512, are also concerned with oligomerisation of feeds containing higher olefin content and they employ particular catalyst systems to achieve this end.
Tubular oligomerisation reactors employing zeolite catalysts typically comprise one or more bundles of tubes also termed “reactor tubes”, mounted, preferably vertically, within a shell. The tubes are packed with the zeolite catalyst typically in the form of pellets and the olefin reactant is passed through the tubes in which it is oligomerised, typically from top to bottom. The length of the tube in industrial practice, is generally from 2 to 15 meters. The diameter of the tube, the thickness of the walls of the tubes and the materials from which the tubes are made are important since oligomerisation reactions are exothermic and it is important to dissipate the heat generated by the oligomerisation reaction. Accordingly relatively small diameter tubes, such as those having an internal diameter from 25 to 75 mm, are preferred, more preferably 35 to 50 mm diameter tubes. The reactor tubes are preferably of high strength material and are thin walled and of a material with a high thermal conductivity. The high strength is required to withstand the high pressures that are generally used in the oligomerisation of olefins in a tubular reactor employing a zeolite catalyst. Duplex stainless steel is a preferred material for manufacture of the tubes.
Any convenient number of tubes may be employed in a reactor shell. Typically, operators use from 10 to 500 tubes per shell, preferably arranged in parallel. Preferred reactors contain about 77 tubes or 180 tubes per shell, although any number may be employed to suit the needs of the operator, e.g. 360 or 420. The tubes are preferably mounted within the shell and a temperature control fluid is provided around the outside of the tubes but within the shell to dissipate heat generated by the exothermic reaction that, in use, takes place within the reactor tubes. One reactor may comprise multiple bundles of tubes, for example up to 7 or 8, or even 9 bundles, and preferably, in use, the temperature of the fluid within the tubes in all the bundles in the same reactor is controlled by means of the same temperature control fluid system.
Reference in this specification to removal of heat from the (reactor) tubes or temperature control of the (reactor) tubes is, in context, intended to mean removal of heat from the materials contained within the tubes where reaction takes place (generally comprising, in use, unreacted feed, reaction products and catalyst) and control of the temperature of those materials contained within the tubes. It will be appreciated that the heat generation on the catalyst and heat removal from the tube wall may cause a radial temperature gradient through the cross-section of the tube, such that the centre of the tube may become significantly hotter than the wall of the tube. One convenient way to remove the heat from the tubes and carry out the temperature control is to generate steam within the reactor on the shell side around the exterior of the tubes. This provides a good heat transfer coefficient on the shell side. If the present invention is performed in a chemical plant or a refinery, the steam generated by the oligomerisation process may be readily integrated into the steam system typically present at such sites. The reaction heat from oligomerisation may then be put to use in another part of the oligomerisation process, or with another process in the plant or the refinery, where heat input is required.
On an industrial scale it is desirable that these tubular reactors can run continuously for as long as possible and that the conversion and selectivity of the reaction is maintained over such extended production runs.
U.S. Pat. No. 6,884,914 relates to the oligomerisation of olefins and provides an olefin feed stream which can be oligomerised at high efficiency. The olefin feed stream may be obtained from oxygenates by treatment with mole sieves. However, although refining feeds may also be used, these are preferably used in admixture with the olefin feed obtained from the oxygenates. The feed preferably contains about 55 wt % olefin and more preferably 60 wt % olefins. U.S. Pat. No. 6,884,914 discusses various different catalysts that can be employed and zeolite catalysts are preferred. The oligomerisation reaction is performed at a temperature from 170° C. to about 300° C. preferably about 170° C. to 260° C. most preferably about 180° C. to about 260° C. Operating pressure is said to be not critical although the process is carried out at about 5 MPa to 10 MPa. In the oligomerisations exemplified in U.S. Pat. No. 6,884,914, a feed containing 64 wt % butenes is oligomerised using a ZSM-22 zeolite catalyst. This reaction will be less exothermic than oligomerisation of propylene or isobutylene over a solid phosphoric acid catalyst.
U.S. Pat. No. 6,884,914 is not, however, concerned with optimising oligomerisation of olefins in a tubular reactor. Tubular reactors are the most efficient for oligomerisation reactions over zeolite catalyst because the reactions are highly exothermic and require precise temperature control.
U.S. Pat. No. 5,672,800 (WO 93/16020) is concerned with the oligomerisation of olefins employing a zeolite catalyst, particularly the zeolite ZSM-22. U.S. Pat. No. 5,672,800 indicates that conversion and catalyst life can be improved if the oligomerisation is performed in the presence of water. The compositions in the examples show a significant improvement when water is present, U.S. Pat. No. 5,672,800 is not however concerned with oligomerisation in tubular reactors and we have found that the use of the water in tubular reactors according to U.S. Pat. No. 5,672,800 can lead to undesirable corrosion of the reactors or downstream equipment, as the water may combine with olefins in the feed and via a complex reaction mechanism lead to the formation of organic acids such as acetic, propionic and/or butyric acids. The maximum conversion achieved using the techniques of U.S. Pat. No. 5,672,800 is 1240 tonnes of oligomer per tonne of catalyst and more typical conversions are considerably less.
As already indicated, the oligomerisation of olefins over zeolite catalyst is a highly exothermic reaction, particularly the oligomerisation of propylene and/or isobutylene. The high temperatures generated can lead to carbonaceous deposits on the catalyst caused by a build up of condensed, heavy hydrocarbons similar to asphalt. Such deposits are commonly termed “coke”, and lead to deactivation of the zeolite catalyst. In general, the higher the concentration of olefin in the feed, the higher will be the rate of heat release from the catalysed reaction, and hence the higher the temperatures reached. Consequently there will be a higher rate of coke formation. This has placed a limit on the maximum concentration of olefin that can be tolerated in the feed.
The ExxonMobil Olefins to Gasoline (EMOGAS) process was described at the Annual Meeting of the National Petrochemical and Refiners Association, 13 to 15 Mar. 2005, at the Hilton Hotel, San Francisco, Calif., USA. The paper described olefin oligomerisation in a tubular reactor employing a zeolite catalyst and specified that the reaction temperature is controlled with water that is fed on the shell side of the reactor. It is stated that the heat released due to EMOGAS reactions in the tubes evaporates water on the shell side. The temperature profile in the tubular reactor is said to be close to isothermal and the temperature is controlled via the shell side water pressure, which controls the temperature of evaporation, and also by the reactor feed temperature. The tubular reactors are said to usually operate at a pressure between 5.5 and 7.6 MPa (800 and 1100 psi) and temperatures around 204° C. (400° F.).
The present invention is concerned with the reduction in the temperature fluctuations along the length of the reactor tube and control of the temperature along the length of the reactor tube in order to enhance the life of the catalyst and the conversion achieved. The life and conversion are assessed as the number of tons of oligomer that can be produced per ton of zeolite catalyst before the olefin conversion falls below an economically acceptable level. The invention is concerned with the conditions to be used to sustain the activity of any zeolite catalyst unlike the EMOGAS paper which specifies a particular catalyst that is more stable. The invention is also concerned to provide conditions that enable extended production runs with feeds high in olefin concentration employing conventional zeolite catalysts.
The composition of the material in the tubular reactor varies as the material flows through, usually down, the reactor tube and begins to react. The olefin will have a lower molecular weight at the beginning (inlet) of the reactor tube, where it is predominantly unreacted light olefins and it will become progressively heavier towards the tube outlet as the light olefins are oligomerised to form higher molecular weight olefins. Excessive temperatures caused by the exotherm of the reaction can coke up the catalyst, which leads to deactivation.
In the operation of a tubular reactor for oligomerisation of olefin feed, with zeolite catalyst in the tubes and a temperature control fluid on the shell side, a temperature profile will be observed over the length of a reactor tube. Conventionally, such operation is performed with the tubular reactor arranged such that the feed inlet is at the top and the reaction product outlet is at the bottom. The following description addresses such an arrangement, but it will be understood that the description applies equally to reactors not in top to bottom arrangement. Thus, the temperature profile initially increases at the inlet of the tube, when reaction heat is generated faster than it can be removed by the temperature control fluid around the tube. As the reactants convert further as they move along the tube and their concentration reduces, the reaction rate reduces and the rate of heat generation reduces. At the same time the temperature in the tube increases, and the heat removal rate increases. The temperature profile then typically goes through a maximum, and then shows a decline further along (down) the tube towards the outlet. As the reaction temperature declines along the tube, also heat removal rate reduces, and the temperature profile may then flatten out before the end of the catalyst bed in the tubes is reached.
With fresh zeolite catalyst, the temperature increase at the initial part (e.g. top) of the tube is sharp, and the temperature profile shows a sharp peak. The fresh catalyst at the initial part (top) of the tube performs most of the reaction. Coke will build up where the temperature is at its highest, which will deactivate the catalyst in that part of the tube. U.S. Pat. No. 5,672,800 seeks to overcome this problem by the addition of water to quench the activity of the catalyst. Without this quench the reaction rate will then reduce due to the catalyst deactivation, and hence the rate of heat generation will reduce, and hence the slope of the temperature increase in that part of the temperature profile declines. The catalyst further along (down) the tube will then see a higher concentration of unreacted reactants, and the reaction rate—and hence heat generation rate—will increase in that part of the tube. In this way the peak in the temperature profile known as “the peak temperature” will move along (down) the tube. In order to compensate for the reduced overall catalyst activity, heat removal is typically reduced by increasing the temperature of the temperature control fluid around the tube. The average temperature in the reactor and the temperature at the outlet of the tube or reactor will thereby be increased as the run progresses. In addition, the temperature of the feed delivered to the tube inlet may be adapted as well. Typically it may be increased to keep as much of the reaction as possible at as early (high) as possible a location in the catalyst bed inside the tube. The peak in the temperature profile therefore may not only move along (down) the tube as a production run proceeds but it may also become less sharp and less pronounced.
The rate of heat generation increases with higher reactant concentration. The peak in the temperature profile is therefore sharper and more pronounced when the olefin concentration in the feed to the reactor is higher. The rate of heat generation is also higher with more reactive reactants, typically with the lighter olefins such as propylene and butenes such as isobutylene. The peak in the temperature profile is therefore also sharper and more pronounced when a higher portion of the available butenes is isobutylene, or when a higher proportion of the olefins fed to the reactor is propylene. In case dienes or acetylenes are present, these are even more reactive and will increase the rate of heat generation, in particular in the upstream part of the zeolite catalyst bed. We have also found that cyclopentene generates the same heat of reaction as pentadiene. The total heat of the reaction also depends on the product produced. The greater the degree of oligomerisation of any particular olefin the higher the heat of reaction, because more monomer molecules will have combined to form the product.
The level of di- and polyunsaturates in the feed is typically controlled to below a maximum allowable level. Preferably, the feed composition is limited to containing no more than 100 ppm by weight of acetylene and/or no more than 500 ppm of the C3 polyunsaturates methylacetylene and propadiene or allene, and/or no more than 2500 ppm or more preferably no more than 1000 ppm of butadiene. The reason for these limitations is the high reactivity and extreme coke forming properties of the di- and poly-unsaturates. We have found that if it is necessary to use feeds containing relatively high levels of polyunsaturates, production may be sustained if the olefin concentration in the feed is reduced accordingly. This keeps the carbon deposition low which would otherwise increase due to the heat generated by the reaction of the higher amounts of polyunsaturates present.
The olefin feed to the tubular reactor is generally a mixture of a reactive olefin and an unreactive diluent, which is typically an alkane. This may have the same carbon number as the olefin. However, it is preferred to have unreactive components present that have a higher carbon number than the feed olefin because of their advantageous effect on phase behaviour in the reactor. The rate of heat generated by the oligomerisation reaction depends upon the concentration of the olefin in the feed. The higher the concentration of olefin the more reactive the feed and the greater the heat that is generated. For example in the operation of tubular reactors employing zeolite catalysts to oligomerise propylene containing feeds it has been found necessary to limit the amount of olefin in the feed. This is because, despite employing cooling systems such as the steam generation mentioned previously, it has not been possible to perform extended continuous runs with feeds containing more than 50 wt % propylene. Typically it has only been possible to employ feeds containing much less than 50 wt % propylene, some processes operating at 40 wt % propylene or less.
The feed streams containing the feed olefins such as C3 and C4 olefins are generally refinery steams derived from steam cracking or catalytic cracking and the composition of the stream will depend upon the raw material from which it is produced and the production technology employed. However, propylene refinery steams typically contain up to 75 or depending on severity even up to 79 wt % propylene with the balance being predominantly propane. Similarly butene refinery steams typically contain up to 70 wt % butenes with the balance being predominantly butanes. The reactivity of the olefins in oligomerisations over zeolite catalysts varies according to the nature of the olefin. However it has not been possible to successfully oligomerise C3 to C6 olefins over extended periods of time in tubular reactors employing a zeolite catalyst if the concentration of propylene in the feed exceeds 50 wt % and generally concentrations below 40 wt % have been employed. This has required the expensive addition of diluent to an olefin containing refinery feed. Typically the diluent may be additional amounts of the alkane found in the refinery feed and/or it maybe provided by recycle of the unreacted material derived from the tubular reactor. The need for diluent not only adds to the expense of the operation but it also reduces the volumetric yield of the reaction with associated economic debits.
There therefore remains a need to oligomerise olefin feeds containing a higher concentration of olefin using a zeolite catalyst over extended production runs in a tubular reactor without undue deactivation or premature failure of the catalyst. It will be appreciated that in large scale industrial processes such as are used for the oligomerisation of olefins, small increases in production (such as 1 to 5% increase) have highly significant benefits. In addition, the ability to increase a run length by an apparently small amount also has highly significant benefits.
Excessive peak temperature will cause coking of the catalyst which adversely impacts the conversion of the catalyst. We have now found that if the temperature and pressure within the reactor tube are controlled to within certain limits extended production runs with high conversion of olefin to oligomer may be achieved using the conventional zeolite oligomerisation catalysts with feeds containing higher levels of olefin. We have also found that if these conditions are maintained the extended runs may be achieved without the need for the presence of water. In this way the corrosion of the reactor can be reduced.
We have now developed an oligomerisation process which is capable of providing such benefits.